CORNWALL — Sister Mary Ellen Wisner, a Newburgh native and Cornwall resident, spent years teaching in Catholic schools, including St. Thomas of Canterbury School in Cornwall-on-Hudson.She’s also an artist, but it’s only now, at 90 years old, that she’s getting recognition for her talent.The Dominican Sisters of Hope are giving Wisner her first art show, in the chapel at their Marian Retreat and Conference Center in Ossining.